The phenomenon of superconductivity was discovered in 1911 by Kammerling and Onnes in mercury. Since that time, the phenomenon has been observed in thousands of materials, elements, alloys, inorganic and organic compound semiconductors. However, these materials are superconducting only below a critical temperature of a few degrees Kelvin. Hence, such superconducting materials required cooling by liquid helium. The attendant high expense limited the use of superconductivity to relatively exotic applications, such as accelerators for high energy physics experiments and the like.
However, Bednorz and Miller announced a dramatic breakthrough with their discovery of superconductivity at much higher temperatures in La.sub.2-x Ba.sub.x CuO.sub.4. Since the publication of their discovery Chu reported superconductivity in YBa.sub.2 Cu.sub.3 O.sub.7. Still others reported even higher critical transition temperatures in other similar materials.
At this point in time, there is no satisfactory theory explaining the phenomenon of superconductivity in this new class of materials which will be referred hereinafter as the ceramic oxides. It is, however, inferred from observations to date, that all these oxide superconductors exhibit a crystal structure which is a close derivative of the pervoskite lattice.
The superconducting oxides which have been found thus far invariably comprise oxides of copper, and usually barium. The third component is a rare earth or group IIIb element in a generally octahedral structure. All of the rare earths, except samarium, as well as yttrium have been found to form superconductors.
The superconducting state is extremely sensitive to the concentration of oxygen. In particular neither YBa.sub.2 Cu.sub.3 O.sub.6 nor YBa.sub.2 Cu.sub.3 O.sub.7.5 are superconducting, however, YBa.sub.2 Cu.sub.3 O.sub.x, where x is between 6.5 and 7.2 and exhibits strong superconducting properties. Similarly, substitution of a few oxygen atoms with fluorine in YBa.sub.2 Cu.sub.3 O.sub.7 transforms the nonsuperconducting material into one whose resistance vanishes at the remarkable temperature of approximately 280.degree. K., according to Ovchinski.
The above implies that the superconductive state of these oxides requires a pervoskite crystal structure, slightly modified to exhibit an excess of electrons and an atomic structure which defines zero resistance pathways for excess electrons to move across the crystal.
The discovery of this new class of superconductors with critical temperatures which are certainly above the boiling point of liquid nitrogen and perhaps ultimately even above the freezing point of water will have a major impact on many technologies. However, before widespread applications can materialize major materials processing issues must be resolved. The present family of oxide superconductors are ceramics, and consequently brittle and generally difficult to form and shape. Moreover, as mentioned above, the superconducting state is sensitive to oxygen content and crystal structure, and is capable of being destroyed by improper processing environments and excessive processing temperatures. The latter parameter is a particularly serious limitation because in order to produce articles such as magnets, circuit elements or conductors, of high integrity, it is desirable to be able to achieve ceramics of high or near theoretical densities and avoid porosity. This has thus far proven to be elusive since adequate densification implies higher processing temperatures than are compatible with maintaining the superconducting state. A further issue is the desirability to produce superconductors of small cross-section in order to suppress the formation of autonomous eddy currents within the conductor.
Thus far, it has been reported, that it has been possible to form ceramic "wire" by coextruding the ceramic and a binder followed by firing. However, the porous product is extremely brittle and delicate.
It has also been recognized that it would be desirable to produce the superconductor in the form of thick layers and thin films. The formation of thin films has also been reported. In particular, superconducting films have been produced by plasma spraying on various substrates. However, it is not believed that these methods can provide films of optimal mechanical characteristics and integrity, high density, good adhesion to substrates, and satisfactory match of structural and thermal expansion parameters between films and substrates to ensure long service life under varying environmental conditions.